1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an encapsulation method of an organic electroluminescent device by which the organic electroluminescent device is protected from moisture and oxygen infiltration, and an organic electroluminescent panel using the same.
2. Description of the Related Art
Recently, attention has been given to electroluminescent display devices as spontaneous emission display devices because they have advantageous features suitable for the next generation display devices, such as a wide viewing angle, a high contrast ratio and a high response speed. Electroluminescent display devices are classified into inorganic electroluminescent display devices and organic electroluminescent display devices according to materials used to form the emitter layers. Organic electroluminescent display devices are in widespread use and exhibit luminance, a low driving voltage and a high response speed.
A general organic electroluminescent display device (hereinafter abbreviated as an organic EL device) is basically configured such that a first electrode layer with a predetermined pattern is formed on a substrate. A hole transport layer, a light emitting layer and an electron transport layer are sequentially stacked on the first electrode. A second electrode layer with a predetermined pattern is formed on the electron transport layer in a direction orthogonal to the first layer. The hole transport layer, the light emitting layer and the electron transport layer are organic layers made from organic compounds. To drive the organic EL device having the above-described configuration, a predetermined voltage is applied between the first electrode and the second electrode. Holes injected from the first electrode move to the light emitting layer via the hole transport layer and electrons from the second electrode are injected into the light emitting layer via the electron transport layer. The electrons and holes are recombined at the light emitting area to generate excitons. The generated excitons de-excite from an excited state to a base state so that fluorescent molecules of the light emitting layer emit light, thereby forming an image.
The organic layers made from organic compounds having low heat resistance are succeptible to deterioration due to moisture, and the second electrode formed on the organic layers may experience deterioration in performance. As a result, when the organic EL device is driven in the presence of air, the electroluminescent characteristics sharply degrade. Thus, in order to obtain an efficient organic EL device, it is necessary to protect the organic layers against exposure to moisture or oxygen by encapsulating the organic layers of the EL device. In order to maintain good performance of an organic EL device, improved encapsulation methods have recently been the subject of active research.
FIG. 1 is a longitudinal cross-sectional view illustrating a conventional method of encapsulating an organic EL device.
Referring to FIG. 1, an organic EL device 30 including a first electrode layer 31, a hole transport layer 32, a light emitting layer 33, an electron transport layer 34 and a second electrode layer 35, is formed on a substrate 10. A box-shaped or dome-shape encapsulation cap 21 having a moisture absorbent layer 41 installed on its internal surface and having an open face, is adhered to the substrate 10 by an adhesive layer 50 in a state in which the organic EL device 30 is positioned on its internal surface. Since only the adhesive layer 50 is used to prevent infiltration of moisture or oxygen, there is a limit to optionally protecting the organic EL device 30 against moisture or oxygen. In order to effectuate good encapsulating capacity of the organic EL device 30 by isolating the same from moisture or oxygen, a cavity 21a defined by the substrate 10 and the encapsulation cap 21 is filled with oxygen- or moisture-free nitrogen such that the pressure of the cavity 21a is higher than atmospheric pressure. This approach, however, involves several problems including an increase in cost, poor manufacturability and increased defect ratio.
FIG. 2 is a longitudinal cross-sectional view illustrating another conventional method of encapsulating an organic EL device 30.
As shown in FIG. 2, an adhesive resin layer 42 is coated on the surface of the organic EL device 30 formed on a substrate 10. An encapsulation plate 22 covers the adhesive resin layer 42 to encapsulate the organic EL device 30. Where the adhesive resin layer 42 is applied to a plurality of organic EL devices, it is difficult to apply the adhesive resin layer 42 to the desired portions only. Accordingly, the adhesive resin layer 42 is often erroneously coated on electrodes 31 and 35 which are to be exposed to the outside and not encapsulated. In this event, it is necessary to remove the coated adhesive resin layer 42 to expose the erroneously coated electrodes 31 and 35. This process results in poor manufacturability and increases the defect ratio for the product. Another problem with this method is that the adhesion process may produce between the adhesive resin layer 42 and the encapsulation layer 22, and which, deteriorates the performance of the organic EL device 30.